Method for stretching polymer film, polarizing film and method for producing the same, polarizer, birefringencial film and liquid crystal display device

ABSTRACT

A method for stretching an optical polymer film is described, which comprises allowing a locus L1 of the holding means from a substantial holding Initiation point to a substantial holding release point on one edge of the polymer film, a locus L2 of the holding means from a substantial holding initiation point to a substantial holding release point on the other edge of the polymer film, and a distance W between the two substantial holding release points to satisfy the following equation (1), maintaining the supporting property of the polymer film, stretching the film in the presence of a state in which the volatile content is 5% or more, and then, decreasing the volatile content while shrinking the film:
 
| L 2− L 1|&gt;0.4 W .  (1)

This application is a divisional of application Ser. No. 09/880,944,filed on Jun. 15, 2001, now U.S. Pat. No. 6,746,633.

FIELD OF THE INVENTION

The present invention relates to a method for orientating an opticalpolymer film by oblique stretching, a method for manufacturing apolarizing film with high yield, a polarizing film, polalizer andbirefringencial film using the optical polymer film obtained by themethod, and a liquid crystal display device using the polarizer.

BACKGROUND OF THE INVENTION

Demand for polarizers has surged with the diffusion of liquid crystaldisplay devices (hereinafter referred to as LCDs). In a polarizer, aprotective film or protective films are generally laminated on one orboth sides of a polarizing layer having polarizing ability through anadhesive layer.

As a raw material for the polarizing layers, polyvinyl alcohol(hereinafter referred to as PVA) has been mainly used. PVA films aredyed with iodine or dichromatic dyes after uniaxial stretching, orstretched after dying, and further crosslinked with boron compounds,thereby forming polarizing films for the polarizing layers.

As the protective films, cellulose triacetate has been mainly used,because it is optically transparent and has low birefringence. Usually,the films are longitudinally uniaxially stretched, so that absorptionaxes of the polarizing films are longitudinally roughly parallel.

In the conventional LCDs, transmission axes of the polarizers arearranged, inclined at 45° to the longitudinal or lateral direction ofimages. Accordingly, in the stamping process of the polarizers producedin the roll form, stamping has been carried out in a direction inclinedat 45° to the longitudinal direction of the rolls.

However, stamping in the direction at 45° generates unusable portions inthe vicinity of ends of the rolls. In particular, large-sized polarizershave the problem of a low yield. Further, for the polarizers afterlamination, it is difficult to recycle materials thereof, whichintroduces the problem of increased waste.

Birefringencial films are used by adhering them to the polarizers or thelike forming the LCDs, for optical compensation such as coloringprevention and enlargement of the angle of visibility, and it is neededto set orientation axes of the birefringencial films at various anglesto the transmission axes of the polarizers. Conventionally, a system ofcutting longitudinally or laterally uniaxially stretched films bystamping out film thereof in such a manner that orientation axes thereofgive specified angles inclined to the edge thereof, which has raised theproblem of a low yield, similarly to the polarizers.

For solving this problem, some methods are proposed in which theorientation axes of polymer films are inclined at desired angles to filmtransferring directions. JP-A-2000-9912 (the term “JP-A” as used hereinmeans an “unexamined published Japanese patent application”) proposesthat a plastic film is longitudinally or laterally stretched at speedsdifferent from each other on the right and left to the stretchingdirection, while uniaxially stretching the film in a lateral orlongitudinal direction different from the above-mentioned stretchingdirection, thereby inclining the orientation axis thereof to theabove-mentioned uniaxial stretching direction. However, according tothis method, when a tenter system is used, for example, it is necessaryto give the difference in transferring speed between the right and leftto the stretching direction. As a result, crease due to unevenstretching stress, wrinkles and local unevenness of film thicknesscaused thereby are developed, resulting in difficulty in obtaining adesired inclined angle (45° in the polarizer). When the difference inspeed between the right and left is intended to be decreased, thestretching stage must be lengthened, which causes installation cost-tobe greatly increased.

Further, JP-A-3-182701 proposes a method for producing a film having astretching axis of any angle θ to a running direction of the film, whichcomprises holding the continuous film at both of its right and leftedges with a plurality of pairs of holding points inclined at the angleθ to the running direction of the film, and stretching the film in the θdirection by means of each pair points with running of the film.However, this method also causes crease and wrinkles to be developed inthe film, because of different running speed between the right and leftedges of the film. For relieving this problem, the stretching stage mustbe lengthened, which causes installation cost to be greatly increased.

Furthermore, JP-A-2-113920 proposes a method for stretching a film in adirection diagonal to a longitudinal direction of the film, whichcomprises running the film while holding it between two rows of chucksrunning on a tenter rail arranged so that the running distances of thechucks within, a specified running section on both sides of the film aredifferent from each other. However, this method is also unfavorable forthe production of an optical film because crease and wrinkles aredeveloped when the film is stretched in the diagonal direction.

SUMMARY OF THE INVENTION

An object of the invention is to provide a diagonal stretching method ofa polymer film, which can improve the yield in a stamping process of apolarizer or a birefringencial film.

Another object of the invention is to provide a high-performance,inexpensive polarizer or birefringencial film comprising a diagonallystretched polymer film obtained by the above-mentioned method.

A still another object of the invention is to provide a liquid crystaldisplay device using the above-mentioned polarizer.

As a result of intensive studies for attaining the above-mentionedobjects, the present inventors have discovered a method of obtainingdiagonal orientation by adjustment of the volatile content in thestretching process and the shrinkage process without development ofcrease, wrinkles and local unevenness of film thickness.

That is to say, according to the present invention, there are provided amethod for stretching a polymer film, a polarizing film, a polarizer, abirefringencial film and a liquid crystal display device each having thefollowing constitution:

1. A method for stretching an optical polymer film by holding both edgesof the continuously supplied polymer film by holding means, andimparting tension thereto while advancing the holding means in alongitudinal direction of the film, which comprises allowing a locus L1of the holding means from a substantial holding initiation point to asubstantial holding release point on one edge of the polymer film, alocus L2 of the holding means from a substantial holding initiationpoint to a substantial holding release point on the other edge of thepolymer film, and a distance W between the two substantial holdingrelease points to satisfy the following equation (1), maintaining thesupporting property of the polymer film, stretching the film in thepresence of a state in which the volatile content is 5% or more, andthen, decreasing the volatile content while shrinking the film:|L2−L1|>0.4W  (1);

(2) The stretching method described in the above (1), wherein L1, L2 andW satisfy the following equation (2):0.9W<|L2−L1|<1.1W  (2);

(3) The stretching method described in the above (1) or (2), wherein thedifference in longitudinal advancing speed between the holding means onboth edges of the polymer film is less than 1%;

(4) The stretching method described in any one of the above (1) to (3),wherein an angle made by a center line of a polymer film introduced-forholding and a center line of a polymer film sent out to a subsequentstep after release of the holding is within 3°;

(5) The stretching method described in any one of the above (1) to (4),wherein the stretch ratio of the polymer film is from 1.2 to 10;

(6) The stretching method described in any one of the above (1) to (5),wherein the polymer film is stretched in the presence of a state inwhich volatile content is 7% or more;

(7) The stretching method described in any one of the above (1) to (5),wherein the polymer film is stretched in the presence of a state inwhich volatile content is 10% or more;

(8) The stretching method described in any one of the above (1) to (5),wherein the polymer film is once stretched at a stretch ratio of 2 to 10in the presence of a state in which volatile content is 10% or more, andthen shrunk 10% or more, thereby inclininganorientation directionofthepolymer film at 40 to 50° to the longitudinal direction thereof;

(9) A method for stretching a continuously supplied optical polymer filmby imparting tension thereto while holding both edges thereof by holdingmeans, which comprises

(i) stretching the film at least in a width direction thereof at astretch ratio of 1.1 to 20.0,

(ii) adjusting the difference in longitudinal advancing speed betweenthe holding means on both edges of the film to 1% or less,

(iii) bending the advancing direction of the film in the state that bothedges thereof are held so that the advancing direction of the film at anoutlet of the step of holding both edges of the film is inclined at anangle of 20 to 70° to a substantial stretching direction of the film,and

(iv) maintaining the supporting property of the polymer film, stretchingthe film in the presence of a state in which the volatile content is 5%or more, and then, decreasing the volatile content while shrinking thefilm,

(10) The stretching method described in any one of the above (1) to (9),wherein the polymer is polyvinyl alcohol, a cellulose acylate, apolycarbonate or a polysulfone;

(11) The stretching method described in any one of the above (1) to (9),wherein the polymer is a vinyl alcohol-based polymer;

(12) A method for producing a polarizing film comprising stretching avinyl alcohol-based polymer by the method described in any one of theabove (1) to (9), and allowing a polarizing element to be adsorbedbefore or after stretching;

(13) A polarizing film produced by the method of the above (12), whereinthe longitudinal direction of the film is inclined at an angle of 20 to70° to a transmission axis direction;

(14) The polarizing film of the above (13), wherein the longitudinaldirection of the film is inclined at an angle of 40 to 50° to thetransmission axis direction;

(15) A polarizer in which at least one side of the polarizing film ofthe above (13) or (14) is protected with a transparent film;

(16) The polarizer of the above (15), wherein the retardation of theprotective film on at least one side at 632.8 nm is 10 nm or less;

(17) A liquid crystal display device in which the polarizer of the above(16) is used as at least one of two polarizers disposed on both sides ofa liquid crystal cell; and

(18) A birefringencial film produced by stretching according to themethod described in any one of the above (1) to (9), in which alongitudinal direction of the film and an orientation direction thereofare inclined at 20 to 70° to a parallel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view for illustrating an example of a methodof the invention in which a polymer film is diagonally stretched;

FIG. 2 is a schematic plan view for illustrating another example of amethod of the invention in which a polymer film is diagonally stretched;

FIG. 3 is a schematic plan view for illustrating still another exampleof a method of the invention in which a polymer film is diagonallystretched;

FIG. 4 is a schematic plan view for illustrating a further example of amethod of the invention in which a polymer film is diagonally stretched;

FIG. 5 is a schematic plan view for illustrating a still further exampleof a method of the invention in which a polymer film is diagonallystretched;

FIG. 6 is a schematic plan view for illustrating a yet still furtherexample of a method of the invention in which a polymer film isdiagonally stretched;

FIG. 7 is a schematic plan view showing a manner in which a conventionalpolarizer is formed by stumping out;

FIG. 8 is a schematic plan view showing a manner in which a polarizer ofthe invention is formed by stamping out; and

FIG. 9 is a schematic plan view showing a layer structure of a liquidcrystal display device of Example 5.

DESCRIPTION OF THE REFERENCE NUMERALS AND SIGNS

-   (A): A direction in which a film is introduced-   (B): A direction in which a film is transferred to a subsequent step-   (a): The step of introducing a film-   (b): The step of stretching a film-   (c): The step of transferring a stretched film to a subsequent step-   A1: A holding position of a film by a holding means and a starting    position of film stretching (a substantial holding initiation point:    right)-   B1: A holding position of a film by a holding means (left)-   C1: A starting position of film stretching (a substantial holding    initiation point: left)-   Cx: A film releasing position and a terminal reference position of    film stretching (a substantial holding release point: left)-   Ay: A terminal reference position of film stretching (a substantial    holding release point: right)-   |L1−L2|: The difference in the travel-between right and left film    holding means-   W: The substantial width of a film at a terminal of a stretching    step-   θ: The angle made by a stretching direction and a running direction    of a film-   11: A center line of a film on an introduction side-   12: A center line of a film sent to a subsequent step-   13: A locus of a film holding means (left)-   14: A locus of a film holding means (right)-   15: A film on an introduction side-   16: A film sent to a subsequent step-   17, 17′: Right and left film holding initiation points-   18, 18′: Release points from right and left film holding means-   21: A center line of a film on an introduction side-   22: A center line of a film sent to a subsequent step-   23: A locus of a film holding means (left)-   24: A locus of a film holding means (right)-   25: A film on an introduction side-   26: A film sent to a subsequent step-   27, 27′: Right and left film holding initiation points-   28, 28′: Release points from right and left film holding means-   33, 43, 53, 63: Loci of film holding means (left)-   34, 44, 54, 64: Loci of film holding means (right)-   35, 45, 55, 65: Films on an introduction side-   36, 46, 56, 66: Films sent to a subsequent step-   71: An absorption axis (stretching axis)-   72: A longitudinal direction-   81: An absorption axis (stretching axis)-   82: A longitudinal direction-   91, 91′: Iodine type polarizers (polarizing layers)-   92: A Lower polarizer-   93: An upper polarizer-   94, 94′: Optical compensated films-   95: An antiglare antireflection film-   96: A protective film (Fujitac)-   97: A liquid crystal cell-   98: A back light

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in detail below.

FIGS. 1 and 2 each shows a schematic plan view for illustrating atypical example of a method of the invention in which a polymer film isdiagonally stretched.

The stretching method of the invention comprises the steps of (a)introducing a raw film in a direction indicated by an arrow (A), (b)stretching the film in a width direction thereof, and (c) transferring astretched film to a subsequent step, that is to say, in a directionindicated by an arrow (B). The term “stretching step” as usedhereinafter means the whole steps for performing the stretching methodof the invention including these steps (a) to (c).

The film is continuously introduced in the direction indicated by (A),and first held at point B1 with a holding means on the left as seen fromthe upstream side. At this moment, the other edge of the film is notheld, so that no tension is developed in the width direction. That is tosay, point B1 does not correspond to the substantial holding initiationpoint of the invention.

In the invention, the substantial holding initiation point is defined bypoints at which both edges of the film are first held. The substantialholding initiation point is indicated by two points of holdinginitiation point A1 on the downstream side and Point C1 at which astraight line roughly perpendicularly drawn from A1 to a center line 11(FIG. 1) or 21 (FIG. 2) of the film on the introduction side intersectsa locus 13 (FIG. 1) or 23 (FIG. 2) of the holding means.

When holding means on both edges are transferred substantially at thesame speed, starting at these points, A1 moves to A2, A3, . . . An foreach time unit, and C1 similarly moves to C2, C3, . . . Cn. That is tosay, a stretching direction at that moment is indicated by a lineconnecting points An and Cn which the standard holding means pass at thesame moment.

In the method of the invention, An is gradually delayed to Cn as shownin FIGS. 1 and 2, so that the stretching direction slowly comes to beinclined from a direction perpendicular to the transferring direction.The substantial holding release point of the invention is defined by twopoints of point Cx on the downstream side at which the film is releasedfrom the holding means and point Ay at which a straight line roughlyperpendicularly drawn from Cx to a center line 12 (FIG. 1) or 22 (FIG.2) of the film transferred to a subsequent step intersects a locus 14(FIG. 1) or 24 (FIG. 2) of the holding means on the opposite side.

The angle of the final stretching direction of the film is determined bythe ratio of the difference Ay−Ax (that is to say, |L1−L2|) in thetravel between the right and left holding means at a substantial endpoint of the stretching step (the substantial holding release point) tothe distance W between the substantial holding release points (thedistance between Cx and Ay) Accordingly, the angle of inclination θ madeby the stretching direction and the transferring direction to thesubsequent step satisfies the following equation:Tan θ=W/(Ay−Ax), that is to say, Tan θ=W/|L1−L2|

Although an upper edge of the film in FIGS. 1 and 2 is held until 18(FIG. 1) or 28 (FIG. 2) also after point Ay, the other edge is not held.Accordingly, no new stretching in the width direction is developed, sothat 18 and 28 do not correspond to the substantial holding releasepoint of the invention.

As described above, in the invention, the substantial holding initiationpoints on both edges of the film are not simple holding points by theholding means on the right and left sides. When the above-mentioneddefinition is more strictly described, the two substantial holdinginitiation points of the invention are each a point at which thestraight line connecting either of the right and left holding points andthe other holding point intersects at about right angles with the centerline of the film introduced into the step of holding the film, and aredefined as points positioned most upstream.

Similarly, in the invention, the two substantial holding release pointsare each a point at which the straight line connecting either of theright and left holding points and the other holding point intersects atabout right angles with the center line of the film sent out to thesubsequent step, and are defined as points positioned most downstream.

The term “about right angles” as used herein means that the angle madeby the center line of the film and the straight line connecting theright and left substantial holding initiation points or substantialholding release points is 90±0.5°.

When it is tried that the difference in the travel between the right andleft holding means is given using a stretching machine of the tentersystem like the invention, a great gap between the holding point held bythe holding means and the substantial holding initiation point, orbetween the release point from the holding means and the substantialholding release point is sometimes developed by mechanical restrictionssuch as the length of a rail. However, so long as the travel between thesubstantial holding initiation point and substantial holding releasepoint defined above satisfies the relationship of equation (1), theobjects of the invention is attained.

In the above, the inclined angle of the orientation axis in theresulting stretched film can be controlled and adjusted by the ratio ofthe difference |L1−L2| in the travel between the right and left holdingmeans to the outlet width W of step (c).

In the polarizers and the birefringencial films, films oriented at 45°to the longitudinal directions are often desired. In this case, forobtaining an orientation angle of nearly 45°, it is preferable tosatisfy the following equation (2):0.9W<|L1−L2|<1.1W  (2)

It is more preferable to satisfy the following equation (3):0.97W<|L1−L2|<1.03W  (3)

Specific examples of structures of the stretching steps are shown inFIGS. 1 to 6 in which the polymer films are diagonally stretchedsatisfying equation (1), and these can be arbitrarily designed, takinginstallation cost and productivity into consideration.

The angle made by the direction (A) in which the film is introduced intothe stretching step and the direction (B) in which the film istransferred to the subsequent step can be any numerical value. From theviewpoint that the total installation area of equipment including stepsbefore and after stretching is minimized, it is preferred that thisangle is as small as possible. The angle is preferably within 3°, andmore preferably within 0.5°. For example, the structures shown in FIGS.1 and 4 can achieve this value.

In the method in which the running direction of the film is notsubstantially changed as described above, it is difficult to obtain anorientation angle of 45° to the longitudinal direction, which ispreferred in the polarizers and the birefringencial films, only byenlarging the distance between the holding means. Then, |L1−L2| can beincreased by providing a step of shrinking the film after once stretchedas shown in FIG. 1.

The stretch ratio is desirably from 1.1 to 10.0, and more desirably from2 to 10. The subsequent shrinkage ratio is desirably 10% or more.Further, it is also preferred that stretching and shrinking areconducted two or more times as shown in FIG. 4, because |L1−L2| can beincreased.

Furthermore, from the viewpoint of minimized installation cost of thestretching step, a fewer bending cycles and a smaller bending angle ofthe locus of the holding means are preferred. From this viewpoint, it ispreferred that the running direction of the film is bent with both edgesof the film held so that the running direction of the film at an outletof the step of holding both edges of the film is inclined at an angle of20 to 70° to the substantial stretching direction of the film as shownin FIGS. 2, 3 and 5.

As an apparatus for stretching the film while holding both edges thereofin the invention, there is preferably used a tenter apparatus as shownin any one of FIGS. 1 to 5. In addition to the conventional twodimensional tenters, a stretching step of helically giving thedifference in the travel of the holding means on both edges as shown inFIG. 6 can also be used.

In the tenter type stretching machine, chains to which clips are fixedmove along rails in many cases. The laterally uneven stretching methodlike the invention results in the deviation of terminal ends of therails at an inlet and outlet of the step, which sometimes causes thefilm not to be held and released at the right and left edges thereof atthe same time, as shown in FIGS. 1 and 2. In this case, the substantialtravel lengths L1 and L2 are not simple distances between the holdingpoints and the release points, but the travel lengths of the portions inwhich both ends of the film are held with the holding means, as alreadystated.

When there is a difference in running speed between the right and leftedges of film at an outlet of the stretching step, wrinkles and localunevenness of film thickness are developed at the outlet of thestretching step. It is therefore desired that the right and left filmholding means are substantially the same as with each other intransferring speed. The difference in transferring speed is preferably1% or less, more preferably less than 0.5%, and most preferably lessthan 0.05%. The term “speed” as used herein means the length of a locusformed by each of the right and left holding means moving per minute. Ina general tenter type stretching machine or the like, the unevenness ofspeed is generated on the order of seconds or less, depending on thecycle of sprocket teeth driving a chain and the frequency of a drivemotor, and an unevenness of several percents is often generated.However, this unevenness of speed does not correspond to the differencein speed described in the invention.

Wrinkles and local unevenness of film thickness are developed accordingto the generation of the difference in the travel between the right andleft holding means. For solving this problem, the invention comprisesmaintaining the supporting property of the polymer film, stretching thefilm in the presence of a state in which the volatile content is 5% ormore, and then, decreasing the volatile content while shrinking thefilm. The term “maintaining the supporting property of the polymer film”as used herein means that the film can be held at both sides withoutimpairment of film properties.

Further, the term “stretching the film in the presence of a state inwhich the, volatile content is 5% or more” does not necessarily meanthat a state in which the volatile content is 5% or more is maintainedthroughout the entire course of the stretching step, but means that thevolatile content may be less than 5% in a part of the stretching step,as long as the stretching at a volatile content of 5% or more expressesthe effect of the invention. Methods for allowing the volatile matter tobe contained in such a form include a method of casting a film, therebyallowing the volatile matter such as water or a nonaqueous solvent to becontained, a method of immersing a film in the volatile matter such-aswater or a nonaqueous solvent, coating a film therewith, or spraying iton a film, before stretching, and a method of coating a film with thevolatile matter such as water or a nonaqueous solvent during stretching.A film of a hydrophilic polymer such as polyvinyl alcohol contains waterin an atmosphere of high temperature and humidity, so that it can beallowed to contain the volatile matter by stretching after moistureconditioning in an atmosphere of high humidity, or by stretching underconditions of high humidity. Other than these methods, any means mayalso be used as long as the volatile content of a polymer film can be 5%or more.

The preferred volatile content varies according to the kind of polymerfilm. The maximum value of the volatile content may be any as long asthe supporting property of the polymer film is maintained. The volatilecontent is preferably from 10% to 100% for polyvinyl alcohol, andpreferably from 10% to 200% for a cellulose acylate.

The stretched film may be shrunk in either of the steps during and afterstretching. Means for shrinking the film include a method of removingthe volatile matter by elevation of temperature. However, any means maybe used as long as the film is shrunk. The volatile content after dryingis preferably 3% or less, more preferably 2% or less, and still morepreferably 1.5% or less.

As described above, a preferred embodiment of the invention is astretching method comprising

(i) stretching the film at least in a width direction thereof at astretch ratio of 1.1 to 20.0,

(ii) adjusting the difference in longitudinal advancing speed betweenthe holding means on both edges of the film to 1% or less,

(iii) bending the advancing direction of the film in the state that bothedges thereof are held so that the advancing direction of the film at anoutlet of the step of holding both edges of the film is inclined at anangle of 20 to 70° to a substantial stretching direction of the film,and

(iv) maintaining the supporting property of the polymer film, stretchingthe film in the presence of a state in which the volatile content is 5%or more, and then, decreasing the volatile content while shrinking thefilm.

A rail for restricting the locus of the holding means in the inventionis often required to have a large bending curvature. Fox avoidinginterference of the film holding means with each other or local stressconcentration caused by sharp bending, it is preferred that the locus ofthe holding means draws a circular arc at a bend.

There is no particular limitation on the polymer film to be stretched inthe invention. Films of appropriate polymers soluble in volatile solventcan be used. Examples of the polymers include PVA, polycarbonates,cellulose acylates and polysulfones.

Although there is no particular limitation on the thickness of the filmbefore stretching, it is preferably from 1 μm to 1 mm, and particularlypreferably from 20 μm to 200 μm, from the viewpoints of the stability offilm holding and the uniformity of stretching.

Although the stretched film of the invention can be used for variouspurposes, it is suitably used as the polarizing film or thebirefringencial film by the characteristic that the orientation axis isinclined to the longitudinal direction. In particular, the polarizingfilm in which the orientation axis is inclined at 40 to 50°, morepreferably 44 to 46°, to the longitudinal direction is preferably usedas a polarizer for an LCD.

When the invention is used for the production of the polarizing film,PVA is preferably used as the polymer. PVA is usually a product obtainedby saponifying polyvinyl acetate. However, it may contain componentscopolymerizable with vinyl acetate, such as unsaturated carboxylicacids, unsaturated sulfonic acids, olefins and vinyl ethers. ModifiedPVA containing acetoacetyl groups, sulfonic acid groups, carboxyl groupsand/or oxyalkylene groups can also be used.

Although there is no particular limitation on the degree ofsaponification of PVA, it is preferably from 80 to 100 mol %, andparticularly preferably from 90 to 100 mol %, from the viewpoint ofsolubility. Further, although there is no particular limitation on thedegree of polymerization of PVA, it is preferably from 1000 to 10000,and particularly preferably from 1500 to 5000.

The polarizing film is obtained by dying PVA, and the dying process isconducted by gas-phase or liquid-phase adsorption. When iodine is usedas an example of the liquid-phase adsorption, a PVA film is immersed inan aqueous solution of iodine-potassium iodide. The amount of iodine ispreferably from 0.1 to 20 g/liter, the amount of potassium iodide ispreferably from 1 to 100 g/liter, and the weight ratio of iodine topotassium iodide is from 1 to 100. The dying time is preferably from, 30to 5000 seconds, and the solution temperature is preferably from 5 to50° C. As the dying method, there can be used any means, such as notonly immersion, but also coating or spraying of an iodine or dyesolution. The dying step may be situated either before or after thestretching step of the invention. However, it is particularly preferredthat the film is dyed in the liquid phase before the stretching step,because the film is properly swelled to result in easy stretching.

Dying with a dichromatic dye as well as iodine is also preferred.Specific examples of the dichromatic dyes include dye compounds such asazo-based dyes, stilbene-based dyes, pyrazolone-based dyes,triphenylmethane-based dyes, quinoline-based dyes, oxazine-based dyes,thiazine-based dyes and anthraquinone-based dyes. The dyes arepreferably water-soluble, but are not limited thereto. Further, it ispreferred that hydrophilic substituent groups such as sulfonic acidgroups, amino groups and hydroxyl groups are introduced into thesedichromatic molecules. Specific examples of the dichromatic moleculesinclude C.I. Direct Yellow 12, C.I. Direct Orange 39, C.I. Direct Orange72, C.I. Direct Red 39, C.I. Direct Red 79, C.I. Direct Red 81, C.I.Direct Red 83, C.I. Direct Red 89, C.I. Direct Violet 48, C.I. DirectBlue 67, C.I. Direct Blue 90, C.I. Direct Green 59 and C.I. Acid Red 37,and further include dyes described in JP-A-1-161202, JP-A-1-172906,JP-A-1-172907, JP-A-1-183602; JP-A-1-248105, JP-A-1-265205 andJP-A-7-261024. These dichromatic molecules are used as free acids,alkali metal salts, ammonium salts or amine salts. A polarizer havingvarious hues can be produced by compounding two or more kinds of thesedichromatic molecules. A polarizing element or polarizer containing acompound (dye) exhibiting black when a polarizing axis intersects atright angles, or containing various kinds of dichromatic molecules so asto show black is preferably excellent in both single plate transmittanceand polarizing rate.

In the course of producing the polarizing film by stretching PVA, anadditive for crosslinking PVA is preferably used. In particular, whenthe diagonal stretching method of the invention is used, theinsufficiently hardened PVA film at the outlet of the stretching stepsometimes causes deviation of the orientation direction of the PVA filmdue to tension given in the step. Accordingly, the film is preferablyimmersed in or coated with a solution of a crosslinking agent in thestep prior to stretching or in the stretching step, thereby allowing thecrosslinking agent to be contained in the film. As the crosslinkingagents, there can be used agents described in U.S. Reissue Pat. No.232,897, and most preferred are boric acid compounds.

The stretching method of the invention is also preferably used in theproduction of a so-called polyvinylene-based polarizing film in whichthe polyene structure is formed by dehydration of PVA or dechlorinationof polyvinyl chloride to obtain polarization caused by conjugated doublebonds.

The polarizing film produced in the invention is used as the polarizerby adhering a protective film or protective films to one or both sidesthereof. There is no particular limitation on the kind of protectivefilm. Examples of the protective films which can be used in theinvention include cellulose esters such as cellulose acetate, cellulosebutyrate and cellulose propionate, polycarbonates, polyolefins,polystyrene and polyesters. However when the retardation value of theprotective film is equal to or more than a specific value, thepolarizing axis and an orientation axis of the protective filmdiagonally deviate from each other, so that linear polarizationunfavorably changes to elliptical polarization. It is thereforepreferred that the retardation value of the protective film is low.

For example, the retardation value is preferably 10 nm or less, and morepreferably 5 nm or less, at 632.8 nm. For obtaining such lowretardation, cellulose triacetate is particularly preferred as a polymerused for the protective film. Further, polyolefins such as Zeonex andZeonor (both manufactured by Nippon Zeon Co., Ltd.) and ARTON(manufactured by JSR Corp.) are also preferably used. In addition, forexample, non-birefringent optical resin materials as described inJP-A-8-110402 or JP-A-11-293116 are also used.

There is no particular limitation on an adhesive used for adhesion ofthe protective layer to the polarizing film. Examples of the adhesivesinclude PVA-based resins (including modified PVA containing acetoacetylgroups, sulfonic acid groups, carboxyl groups and/or oxyalkylene groups)and aqueous solutions of boron compounds. Of these, PVA-based resins arepreferred. The thickness of the adhesive layer is preferably from 0.01to 10 μm, and particularly preferably from 0.05 to 5 μm, after drying.

FIG. 7 shows an example of the stamping of a conventional polarizer, andFIG. 8 shows an example of the stamping of a polarizer of the invention.In the conventional polarizer, an absorption axis 71 of polarization,that is to say, a stretching axis, agrees with a longitudinal direction72, as shown in FIG. 7, whereas in the polarizer of the invention, anabsorption axis 81 of polarization, that is to say, a stretching axis,is inclined at 45° to a longitudinal direction 82, as shown in FIG. 8.This angle agrees with an angle made by an absorption axis of thepolarizer laminated on a liquid crystal cell in an LCD and alongitudinal or lateral direction of the liquid crystal cell itself,which necessitates no diagonal stamping in the stamping step. Moreover,as seen from FIG. 8, the polarizer of the invention is cut linearlyalong the longitudinal direction, so that it can also be produced byslitting along the longitudinal direction without stamping. Accordingly,the polarizer of the invention is also remarkably excellent inproductivity.

From the viewpoint of increasing a contrast of the liquid crystaldisplay device, it is preferred that the polarizer of the invention hasa higher transmittance and a higher degree of polarization. Thetransmittance is preferably 30% or more, and more preferably 40% ormore, at 550 nm. The degree of polarization is preferably 95.0% or more,more preferably 99.0%, or more, and particularly preferably 99.9% ormore, at 550 nm.

The stretched film according to the invention is characterized bydiagonal orientation to the longitudinal direction, so that it can alsobe suitably used as the birefringencial plate. When the film is used asthe birefringencial plate, it is preferably a film obtained bystretching a material excellent in transparency, for example, apolycarbonate, a polysulfone or a cellulose acylate such as celluloseacetate. Of these, a cellulose acylate is particularly preferred.Although there is no particular limitation on the thickness of the film,it is generally from 5 to 300 μm.

The invention will be described in detail with reference to thefollowing examples, but is not limited thereto.

EXAMPLE 1

A PVA film was immersed in an aqueous solution of 5.0 g/l iodine and10.0 g/l potassium iodide at 25° C. for 90 seconds, and further immersedin an aqueous solution of 10 g/l boric acid at 25° C. for 60 seconds.Then, the film was introduced into a tenter type stretching machinehaving the form of FIG. 1, and once stretched at a stretch ratio of 7.0in an atmosphere of 60° C. and 90% RH. Thereafter, the film was shrunkto 5.3 times, and dried at 70° C. while keeping the width thereofconstant. Then, the film was taken out of the tenter. The volatilecontent of the PVA film before start of stretching was 31%, and thatafter drying was 1.5%.

The difference in transferring speed between right and left tenter clipswas less than 0.05%, and the angle made by a center line of the filmintroduced and a center line of the film sent to the subsequent step was0°. Here, |L1−L2| was 0.7 m, W was 0.7 m, and the relationship of|L1−L2|=W was satisfied. Wrinkles and local unevenness of film thicknesswere not observed at an outlet of the tenter.

Further, the above-mentioned stretched film was laminated with asaponified Fujitac film manufactured by Fuji Photo Film Co., Ltd.(cellulose triacetate, retardation value: 3.0 nm) using a 3% aqueoussolution of PVA (PVA-117H manufactured by Kuraray Co., Ltd.) as anadhesive. The resulting laminate was dried at 80° C. to obtain apolarizer having an effective width of 650 mm. A direction of anabsorption axis of the resulting polarizer was inclined at 45° to alongitudinal direction thereof The transmittance of this polarizer at550 nm was 43.3%, and the degree of polarization thereof was 99.98%.

Then, the polarizer was cut to a size of 310×233 mm as shown in FIG. 8.As a result,a polarizer with the absorption axis inclined at 45° to aside could be obtained at an area efficiency of 91.5%.

EXAMPLE 2

A PVA film was immersed in an aqueous solution of 2.0 g/l iodine and 4.0g/l potassium iodide at 25° C. for 240 seconds, and further immersed inan aqueous solution of 10 g/l boric acid at 25° C. for 60 seconds. Then,the film was introduced into a tenter type stretching machine having theform of FIG. 2, and stretched at a stretch ratio of 5.3. The tenter wasbent to a stretching direction as shown in FIG. 2. Thereafter, the filmwas dried in an atmosphere of 80° C. while keeping the width thereofconstant and allowing it to be shrunk. Then, the film was taken out ofthe tenter. The volatile content of the PVA film before start ofstretching was 31%, and that after drying was 1.5%.

The difference in transferring speed between right and left tenter clipswas less than 0.05%, and the angle made by a center line of the filmintroduced and a center line of the film sent to the subsequent step was46°. Here, |L1−L2| was 0.7 m, W was 0.7 m, and the relationship of|L1−L2|=W was satisfied. A substantial stretching direction Ax-Cx at anoutlet of the tenter was inclined at 45° to the center line 22 of thefilm sent to the subsequent step. Wrinkles and local unevenness of filmthickness were not observed at the outlet of the tenter.

Further, the above-mentioned stretched film was laminated with asaponified Fujitac film manufactured by Fuji Photo Film Co., Ltd.(cellulose triacetate, retardation value: 3.0 nm), using a 3% aqueoussolution of PVA (PVA-117H manufactured by Kuraray Co., Ltd.) as anadhesive. The resulting laminate was dried at 80° C. to obtain apolarizer having an effective width of 650 mm.

A direction of an absorption axis of the resulting polarizer wasinclined at 45° to a longitudinal direction thereof. The transmittanceof this polarizer at 550 nm was 43.7%, and the degree of polarizationthereof was 99.97%. Then, the polarizer was cut to a size of 310×233 mmas shown in FIG. 8. As a result, a polarizer with the absorption axisinclined at 45° to a side could be obtained at an area efficiency of91.5%.

EXAMPLE 3

The following solid components were dissolved in a mixed solventconsisting of 92 parts by weight of dichloromethane and 8 parts byweight of methanol to prepare a concentrated solution.

Cellulose triacetate: 89 parts by weight

Triphenyl phosphate: 7.39 parts by weight

Biphenyldiphenyl phosphate: 3.60 parts by weight

Silica: 0.01 parts by weight

The solid concentration of the concentrated solution was 18.2%. Thisconcentrated solution was cast on an endless band, dried until itexhibited the self supporting property, and released as a film.

This film was introduced at a volatile content of 32% into a tenterhaving the form of FIG. 3, and stretched 20% in a width directionthereof. Then, the tenter was bent at 30° to a direction in which thefilm was introduced, as shown in FIG. 3. Thereafter, the film was driedby blowing hot air of 145° C. thereon while keeping the width thereofconstant and allowing it to be shrunk. Then, the film was taken out ofthe tenter. The volatile content after drying was 0.5%.

The difference in transferring speed between right and left tenter clipswas less than 0.05%, and the angle made by a center line of the filmintroduced and a center line of the film sent to the subsequent step was30°. Here, |L1−L2| was 0.29 m, W was 0.5 m, and the relationship of|L1−L2|=0.58W was satisfied. Wrinkles and local unevenness of filmthickness were not observed at an outlet of the tenter, and the volatilecontent at the outlet was 8%. A substantial stretching direction at theoutlet of the tenter was inclined at 60° to the center line of the filmsent to the subsequent step. The film was further dried until thevolatile content was decreased to 1%. The retardation of the resultingfilm was 23 nm, and a slow axis was inclined at 60° to a longitudinaldirection of the film.

EXAMPLE 4

A PVA film was immersed in an aqueous solution of 1.0 g/l iodine and60.0 g/l potassium iodide at 25° C. for 30 seconds, and further immersedin an aqueous solution of 40 g/l boric acid and 30 g/l potassium iodideat 25° C. for 120 seconds. Then, the film was introduced into a tentertype stretching machine having the form of FIG. 4, and stretched at astretch ratio of 2 in an atmosphere of 60° C. and 90% RH. The tenter wasbent to a stretching direction as shown in FIG. 3, and shrinking wasrepeated. Thereafter, the film was dried in an atmosphere of 80° C., andthen, the film was taken out of the tenter. The volatile content of thePVA film before start of stretching was 31%, and that after drying was1.5%.

The difference in transferring speed between right and left tenter clipswas less than 0.05%, and the angle made by a center line of the filmintroduced and a center line of the film sent to the subsequent step was0°. Here, |L1−L2| was 0.7 m, W was 0.7 m, and the relationship of|L1−L2|=W was satisfied. A substantial stretching direction Ax-Cx at anoutlet of the tenter was inclined at 45° to the center line of the filmsent to the subsequent step. Wrinkles and local unevenness of filmthickness were not observed at the outlet of the tenter.

Further, the above-mentioned stretched film was laminated with asaponified Fujitac film manufactured by Fuji Photo Film Co., Ltd.(cellulose triacetate, retardation value: 3.0 nm), using a 3% aqueoussolution of PVA (PVA-117H manufactured by Kuraray Co., Ltd.) as anadhesive. The resulting laminate was dried at 80° C. to obtain apolarizer having an effective width of 650 mm.

A direction of an absorption axis of the resulting polarizer wasinclined at 45° to a longitudinal direction thereof The transmittance ofthis polarizer at 550 nm was 43.7%, and the degree of polarizationthereof was 99.97%. Then, the polarizer was cut to a size of 310×233 mmas shown in FIG. 8. As a result, a polarizer with the absorption axisinclined at 45° to a side could be obtained at an area efficiency of91.5%.

EXAMPLE 5

A PVA film was immersed in an aqueous solution of 1.0 g/l iodine and60.0 g/l potassium iodide at 25° C. for 30 seconds, and further immersedin an aqueous solution of 40 g/l boric acid and 30 g/l potassium iodideat 25° C. for 120 seconds, followed by drying of the PVA film to avolatile content of 2%. Then, the film was introduced into a tenter typestretching machine having the form of FIG. 2, and stretched at a stretchratio of 5.3 in an atmosphere of 60° C. and 90% RH. The tenter was bentto a stretching direction as shown in FIG. 2. Thereafter, the film wasdried in an atmosphere of 80° C. while keeping the width thereofconstant and allowing it to be shrunk. Then, the film was taken out ofthe tenter. The volatile content of the PVA film during stretching in anatmosphere of 60° C. and 90% RH was 19%, and that after drying was 1.0%.

The difference in transferring speed between right and left tenter clipswas less than 0.05%, and the angle made by a center line of the filmintroduced and a center line of the film sent to the subsequent step was46°. Here, |L1−L2| was 0.7 m, W was 0.7 m, and the relationship of|L1−L2|=W was satisfied. A substantial stretching direction Ax-Cx at anoutlet of the tenter was inclined at 45° to the center line 22 of thefilm sent to the subsequent step. Wrinkles and local unevenness of filmthickness were not observed at the outlet of the tenter.

Further, the above-mentioned stretched film was laminated with asaponified Fujitac film manufactured by Fuji Photo Film Co., Ltd.(cellulose triacetate, retardation value: 3.0 nm), using a 3% aqueoussolution of PVA (PVA-117H manufactured by Kuraray Co., Ltd.) as anadhesive. The resulting laminate was dried at 80° C. to obtain apolarizer having an effective width of 650 mm.

A direction of an absorption axis of the resulting polarizer wasinclined at 45° to a longitudinal direction thereof. The transmittanceof this polarizer at 550 nm was 43.7%, and the degree of polarizationthereof was 99.97%. Then, the polarizer was cut to a size of 310×233 mmas shown in FIG. 8. As a result, a polarizer with the absorption axisinclined at 45° to a side could be obtained at an area efficiency of91.5%.

COMPARATIVE EXAMPLE 1

A commercial iodine polarizer (HLC 2-5618, width: 650 mm, manufacturedby Sanritz Corporation) was used as a polarizer for ComparativeExample 1. This polarizer was cut so as to provide an absorption axisinclined at 45° to a side as shown in FIG. 7, resulting in an areaefficiency of 64.7%.

COMPARATIVE EXAMPLE 2

A PVA film was immersed in an aqueous solution of 2.0 g/l iodine and 4.0g/l potassium iodide at 25° C. for 240 seconds, and further immersed inan aqueous solution of 10 g/l boric acid at 25° C. for 60 seconds, inthe same manner as, with Example 2, followed by drying at 80° C. for 10minutes. The volatile content of the PVA film was 1%. Then, the driedfilm was introduced into a tenter type stretching machine having theform of FIG. 2, and stretched at a stretch ratio of 5.3. The tenter wasbent to a stretching direction as shown in FIG. 2. Thereafter, the filmwas dried in an atmosphere of 80° C. while keeping the width thereofconstant. Then, the film was taken out of the tenter. Wrinkles remainedall over the film, which made it quite impossible to use the film as anoptical film.

EXAMPLE 6

(Preparation of Optical Compensated Film)

Water (130 g) and 40 g of methanol were added to 30 g of straight-chainalkyl-modified PVA (MP-203, manufactured by Kuraray Co., Ltd.) todissolve it by stirring, followed by filtration through a polypropylenefilter having a pore size of 30 μm. Thus, a coating solution for anorientation layer was prepared.

A 100-μm thick triacetyl cellulose film (manufactured by Fuji Photo FilmCo., Ltd.) having an undercoat layer of a thin gelatin film (0.1 μm) wascoated with the above-mentioned coating solution for the orientationlayer by use of a bar coater, and dried at 60° C. Then, the rubbingtreatment was conducted in a direction inclined at 45° to the machinedirection to form the orientation layer having a thickness of 0.5 μm.

Then, 1.6 g of compound LC-1 having the following structure as a liquidcrystal discotic compound, 0.4 g of phenoxydiethylene glycol acrylate(M-101, manufactured by Toagosei Chemical Industry Co., Ltd.), 0.05 g ofcellulose acetate butyrate (CAB 531-1, manufactured by Eastman ChemicalCo.) and 0.01 g of a photopolymerization initiator (Irugacure 907,manufactured by Ciba Specialty Chemicals Inc.) were dissolved in 3.65 gof methyl ethyl ketone, and the resulting solution was filtered througha polypropylene filter having a pore size of 1 μm to prepare a solutionfor an optical anisotropy layer.

The above-mentioned coating solution for the optical anisotropy layerwas applied onto the above-mentioned orientation layer with a barcoater, and dried at 120° C. Then, heating was further conducted for 3minutes to age liquid crystals, thereby orienting the discotic compound.Then, the coating layer was irradiated with ultraviolet at anirradiation intensity of 400 mW/cm² using a 160-W air-cooling metalhalide lamp (manufactured by Eye Graphics Co., Ltd.), as kept at 120°C., so as to give an irradiation energy amount of 300 mJ/cm², hardeningthe coating layer to form the optical anisotropy layer having athickness of 1.8 μm, thereby preparing an optical compensated film.

Then, as shown in FIG. 9, the optical compensated film 94′ was providedon one side of one polarizing film 91′ of two iodine polarizing films 91and 91′ prepared in Example 2, and a saponified Fujitac film 96manufactured by Fuji Photo Film Co., Ltd. (cellulose triacetate,retardation value: 3.0 nm) was laminated on the other side thereof toprepare a polarizer 92. The optical compensated film 94 was provided onone side of the other polarizing film 91, and a commercial antiglareantireflection film (manufactured by Sanritz Corporation) 95 wasprovided on the other side thereof to prepare a polarizer 93. At thistime, the optical compensated film was laminated so that a rubbingdirection of the orientation layer agreed with a stretching direction ofa polarizing layer.

Using the polarizer 92 as a polarizer on the side of a back light 98 ofthe two polarizers between which a liquid crystal cell 97 of an LCD wasplaced, and the polarizer 93 as a polarizer on the display side, bothwere laminated with the liquid crystal cell 97 at the optical anisotropylayer sides of the optical compensated films 94 and 94′ through anadhesive to prepare the LCD.

The LCD thus prepared indicated excellent luminance, visual field anglecharacteristics and visibility, and even the use thereof at 40° C. at30% RH for 1 month showed no deterioration of display quality.

(Measurement of Transmittance and Degree of Polarization at 550 nm)

The transmittance was measured with a Shimazu recordingspectrophotometer UV2100. Further, taking as H0 (%) the transmittancemeasured when two polarizers were overlaid with each other, withabsorption axes agreeing with each other, and taking as H1 (%) thetransmittance measured when two polarizers were overlaid with eachother, with absorption axes crossing at right angles, the degree ofpolarization P (%) was determined by the following equation:P=((H0−H1)/(H0+H1))^(1/2)×100(Measurement of Retardation)

The retardation was measured at 632.8 nm with a KOBRA 21DH manufacturedby Oji Keisoku Kiki Co., Ltd.

The polarizing films, polarizers and birefringencial films produced fromthe polymer films diagonally stretched by the stretching methods of theinvention give high yields in the stamping step, and are simplyobtained. They are therefore provided at low cost. Thus, the liquidcrystal display devices excellent in display quality are provided at lowcost.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

1. A polarizer comprising a polarizing film produced by a methodcomprising stretching a vinyl alcohol-based polymer by a method forstretching an optical polymer film, and allowing a polarizing element tobe adsorbed before or after stretching, wherein the method forstretching an optical polymer film by holding both edges of thecontinuously supplied polymer film by holding means, and impartingtension thereto while advancing said holding means in a longitudinaldirection of the film, comprises allowing a locus L1 of the holdingmeans from a substantial holding initiation point to a substantialholding release point on one edge of the polymer film, a locus L2 of theholding means from a substantial holding initiation point to asubstantial holding release point on the other edge of the polymer film,and a distance W between the two substantial holding release points tosatisfy the following equation (1), maintaining the supporting propertyof the polymer film, stretching the film in the presence of a state inwhich the volatile content is 5% or more, and then, decreasing thevolatile content while shrinking the film:|L2−L1|>0.4W  (1), wherein the longitudinal direction of the film isinclined at an angle of 20 to 70° to a transmission axis direction,wherein at least one side of the polarizing film is protected with atransparent film, wherein the retardation of the transparent film on atleast one side at 632.8 nm is 10 nm or less.
 2. The polarizing film ofclaim 1, wherein the longitudinal direction of the film is inclined atan angle of 40 to 50° to the transmission axis direction.
 3. A liquidcrystal display device in which the polarizer of claim 1 is used as atleast one of two polarizers disposed on both sides of a liquid crystalcell.